1. Field of the Invention
This invention relates to fuel ignition control systems of the intermittent pilot type, and more particularly, to control arrangements for use in such systems for providing fail-safe control of fuel valves of the systems.
2. Description of the Prior Art
In known fuel ignition systems of the pilot ignition type, a pilot valve is operated to supply fuel to a pilot outlet for ignition by sparks provided by a suitable igniter to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve to supply fuel to a main burner for ignition by the pilot flame.
Typically, the operation of the main valve is controlled by a relay of the flame sensing circuit which is operated by the flame sensing circuit when a pilot flame is established, to close its contacts to connect the main valve to an energizing circuit to permit the main valve to operate. However, for a failure of the flame sensing circuit which permits the relay to be operated in the absence of a flame, the main valve will be connected to the energizing circuit, permitting fuel to emanate from the main burner unburned.
Accordingly, various interlock arrangements have been proposed in the prior art, as exemplified by the U.S. Pat. Nos. 3,449,055 to J. C. Blackett, 3,644,074 to P. J. Cade and 3,705,783 to J. S. Warren, in which the fuel valves of the system can be energized only if the flame relay is initially deenergized. In the patented systems, the energization of the pilot valve is effected in response to the operation of a control relay which can be energized only if the flame relay is deenergized. Thereafter, the main valve is energized in response to the operation of the flame relay when a pilot flame is established, and only if the control relay is energized.
While such interlock circuits guard against the welded contact failure referred to above, it appears that the control (or flame) relay may be energized inadvertantly following a failure of a solid state control device of the electronic circuits, allowing the main valve to operate in in the absence of a pilot flame.
In may U.S. Pat. No. 4,035,134, which was issued on July 12, 1977 and assigned to the assignee of the present application, there is disclosed a proven pilot fuel ignition system including a control arrangement which provides an interlock on start-up to prevent the energization of fuel valves of the system under certain failure conditions, including a component failure in the flame sensing circuit and welded contacts of the flame relay. The control arrangement also permits recycling of the system following a momentary power loss or a flame out condition. Other fuel ignition systems which include interlock arrangements are disclosed in my U.S. Pat. Nos. 4,047,878 issued Sept. 13, 1977; 4,087,230 issued May 2, 1978 and 4,077,762, issued Mar. 7, 1978, all of which are assigned to the assignee of the present application.
While such interlock arrangements afford a degree of protection against an unsafe failure of the flame sensing circuit, it would be more desirable if the fail-safe protection were afforded by the flame sensing circuit itself and such interlock arrangement, if desired, be used as back-up safety control for the system.
In the Great Britain Pat. No. 1,334,245, granted to Honeywell Inc. on Oct. 17, 1973, there is disclosed a direct ignition fuel burner control which affords solid state control of the operation of a fuel valve and an igniter circuit. The fuel valve is controlled directly by an SCR device which is enabled by a timing circuit including a FET device and a capacitor. The FET device is maintained pinched-off during the trial for ignition, or when a flame is established, permitting the capacitor to be charged over a first circuit path including the FET device and a diode, and then discharged over a second circuit path which is connected to the gate of the SCR device, causing the SCR device to maintain the fuel valve in fuel supplying condition. If a flame fails to be established during the trial for ignition, the FET device prevents charging of the capacitor causing the SCR device to deactivate the valve.
While this arrangment eliminates the need for relays and affords a degree of fail-safe operation, it appears that under certain failure conditions, the SCR device could be enabled causing the valve to be operated after the trial for ignition interval and in the absence of a flame.
A further consideration is that in most systems, the igniter circuit is disabled by the flame relay. Thus, under certain failure conditions, inadvertant operation of the flame relay may permit fuel to be supplied to the burner apparatus while the igniter is disabled, an undesireable condition.
In the burner control disclosed in the Honeywell Patent, the igniter circuit includes a relaxation oscillator and a control circuit having an FET device which responds to a flame signal to disable the relaxation oscillator when a flame is established. Although the igniter circuit is disabled by a flame signal, the enabling of the igniter circuit is dependent upon the operation of the SCR device which controls the valve, and thus fault conditions of the valve control circuit may affect the operation of the igniter circuit.
Many known fuel ignition control systems operate from a 24 VAC supply, but require 100 VAC for the flame sensing circuit. In such systems, a step-up power transformer is needed to provide isolation of ground and the high voltage for the flame sensing circuit. However, the use of such transformer adds cost to the system.
In the U.S. Pat. No. 3,986,813 to William Hewitt, which issued on Oct. 19, 1976, there is shown an intermittant pilot igniter and valve controller for a gas burner which operates from a 24 VAC source without the need for a power step-up transformer. The controller includes a solid state control circuit which controls a relay for operation of a main valve and an igniter. The control circuit includes an FET device which controls the charging and discharging of a capacitor for effecting the operation of the relay in a manner similar to the circuit disclosed in the British patent referenced above. Although the circuit shown by Hewitt eliminates the need for a power transformer, a step-up transformer is required to supply 48 VAC to the flame sensor, and to provide the proper phase relationship between the voltage on the flame sensor probe and the AC supply.